Field of the Invention
The present invention relates to a light guide including a protrusion portion for alignment and an image reading apparatus including the light guide.
Description of the Related Art
An image reading apparatus which uses an illumination unit including a light-emitting diode (LED) light source and a light guide element is known. Conventionally, as the illumination unit, configurations illustrated in FIGS. 8A and 8B and 9 have been known.
A plurality of LED elements 10 is disposed on an LED substrate 11 in a line. A light guide element 12 is fixed to the LED substrate 11 on which the plurality of LED elements 10 is mounted. Further, the light guide element 12 is aligned by positioning bosses 18a, 18b, and 18c, which are provided on a bottom surface of the light guide element, and fixed to a housing member 21 which is formed above an optical unit 20.
Light emitted from the LED element 10 is propagated through the light guide element 12 and irradiates a document 22 on a document positioning glass 23. Reflected light from the document 22 is guided to an imaging unit 25 by folding mirrors 24a, 24b, 24c, and 24d which are held in the optical unit 20. Further, an image of the document is formed on a solid-state image sensor 26 by the imaging unit 25. The solid-state image sensor 26 photoelectrically converts incident light reflected from the document and outputs image data which indicates the image of the document. The optical unit 20 moves in a reader unit to read an image of the entire document.
As discussed in U.S. Patent Application Publication No. 2010/0232834, the alignment of the light guide element is performed by fitting a plurality of positioning bosses provided on the bottom surface of the light guide element into fitting holes provided in the housing member.
In a device including a configuration illustrated in FIGS. 8A and 8B and 9, most of light flux incident from the LED element 10 to the light guide element 12 is propagated through a light guide portion 14, is deflected by a reflection portion 15, and then exits toward the document 22. However, in the positioning bosses 18a, 18b, and 18c provided on the bottom surface of the light guide element, the propagated light flux is not totally reflected and a part of the light flux is leaked from the positioning bosses. As a result, as illustrated in FIG. 10, the distribution of the light amount in a reading width direction is not uniform.
In a light guide element discussed in U.S. Patent Application Publication No. 2010/0232834, the positioning bosses are disposed between an LED element and an LED element in a longitudinal direction of the light guide element 12. In other words, the positioning bosses of the light guide element are provided in a portion where the light flux is sparse in the longitudinal direction. With this configuration, the leakage of the light from the positioning bosses is reduced. However, it is required to further reduce the leakage of the light from the positioning bosses. Further, since the positions of the positioning bosses depend on the positions of the LED elements, each time the number or the arrangement of LED elements is changed, the light guide element needs to be newly created, which increases the cost.
Further, as a measure for preventing the non-uniform distribution of the light amount due to the leakage of the light flux from the positioning bosses, as illustrated in FIG. 11, a measure for providing the positioning bosses outside effective ranges 108d and 108e of the light guide element is considered. However, when the above-mentioned method is used, the entire length of the light guide element is increased, which is an obstacle to reduction in the size of the device.